The present invention relates to a method for producing an optical coupler and/or an optical splitter between a polarization maintaining optical fibre and an optical fibre which is not polarization maintaining, and a corresponding optical coupler and/or splitter.
Polarization maintaining fibres (PMF) are characterized in that they retain the polarization of the input signal within them throughout their length, if this polarization is orientated in one of the two polarization directions of the fibre.
In particular, these fibres have two principal axes of propagation of the optical signal within them (axes of birefringence), called the slow axis and the fast axis. The said axes are essentially perpendicular to each other and have different characteristics. The fast axis has an effective refractive index which is considerably lower than that of the slow axis, and therefore allows the light beam polarized in the same direction to travel along the fibre at a higher phase velocity than that of the optical beam polarized in the direction of the slow axis. In both directions, however, the output signal from the fibre is kept essentially unaltered. A polarization maintaining fibre is characterized in that it has a difference of more than 10xe2x88x924 between the refractive indices between the two axes of birefringence.
For the purposes of the present invention, the term xe2x80x9cstandard fibrexe2x80x9d denotes an optical fibre which is not polarization maintaining. Preferably, the said standard fibre is a single-mode fibre, particularly a step-index optical fibre.
In particular, in a coupler according to the present invention, an optical signal which passes along a polarization maintaining fibre (xe2x80x9cprincipal fibrexe2x80x9d) is partially extracted from the said fibre and injected into a standard fibre (xe2x80x9cextraction fibrexe2x80x9d). This coupler can be used in applications which do not require a defined polarization state in the extracted signal.
A typical application of this coupler is the monitoring of the optical power carried by a polarization maintaining fibre in which the power measurement is carried out by means of a polarization-insensitive photodetector which is directly connected to the extraction fibre.
More generally, the monitoring of the optical power is carried out by means of any apparatus which does not require the use of any component characterized by a polarization-dependent behaviour. Conversely, in this application, the output signal from the principal fibre must have a polarization state which is nominally identical to that of the input to the fibre.
An optical signal transmitted by a polarization maintaining fibre is characterized not only by the optical power associated with it, but also by the extinction ratio of the polarization, which quantifies the degradation of the state of polarization with respect to that which should theoretically be propagated. In the case of nominally linear polarization, the extinction ratio is defined thus:                     ER        =                              P            C                                P            N                                              (        1        )            
where:
PC is the optical power associated with the axis of birefringence of the fibre along which the optical signal is sent, called the co-polarized component of the optical signal;
PN is the optical power associated with the axis of birefringence of the fibre along which the optical signal is not sent, called the cross-polarized component (theoretically zero).
At the input of the polarization maintaining fibre, the said ratio is infinite. since all the optical power is injected along an axis of birefringence. Subsequently, a small part of the optical power is transferred along the other axis of birefringence, as a result of a possible degradation of the fibre, and consequently the extinction ratio assumes a finite value.
A technology which can be used to produce a coupler and/or splitter between a polarization maintaining fibre and a standard fibre is that of micro-optics.
Splitters made by this technology are marketed, for example, by JDS (Nepean. ON, Canada), E-Tek (San Josxc3xa9, Calif., USA) and Micro-Optics (Hackettstown, N.J., USA). With this technology, the principal fibre and the extraction fibre are both terminated with suitable lenses to form collimators between which an optical power divider element is inserted on each of the two fibres.
In these splitters, one of the two fibres may be a standard fibre, and therefore by suitably selecting the splitting ratio, or the coupling ratio (CR) of the optical power divider element inserted between the collimators, it is possible to extract a portion of the signal from the polarization maintaining fibre and to send it along the standard fibre.
The Applicant has observed that the coupling efficiency between collimated beams associated with different fibres (such as polarization maintaining fibres and standard fibres) is less than 100%, and the achieving of relatively low losses for a component made by micro-optics technology makes the assembly difficult and expensive.
There are also known optical couplers and/or splitters between polarization maintaining fibres produced by the fusion technology, in other words optical couplers and/or splitters in which both the principal fibre and the extraction fibre are polarization maintaining optical fibres. A component of this type is used in all applications in which polarization maintaining division of optical power is desired (the extraction fibre also maintains the polarization).
U.S. Pat. No. 5,224,977 describes an apparatus for producing a fused optical coupler between polarization maintaining fibres. The apparatus comprises. in particular, a first polarization maintaining fibre and a second polarization maintaining fibre, means for holding stripped portions of both of the said fibres in a position of mutual alignment and in contact with each other. The apparatus also comprises means for heating the said portions of both fibres, placed adjacent to the said means for holding stripped portions of both of the said fibres. The said means for heating the portions of both fibres oscillate along the said stripped portions with an amplitude decreasing in steps until the fusion of the said two portions is complete.
The essential aspect which distinguishes the production of a fused optical coupler between polarization maintaining fibres from the production of a similar splitter with standard fibres is that, in the first case, the polarization is desired to be maintained in both of the maintaining fibres. It is therefore necessary to use a method which ensures that the two fibres in the coupling area are fused while maintaining the parallel orientation of the axes of birefringence. The Applicant has observed that this method applied to a coupler considerably complicates the process of fabricating the coupler by comparison with the process used in the case of standard fibres; this is because these methods require the pre-alignment of the axes of birefringence of the fibres, visually or by monitoring the polarization state at the output of the fibres, before fusion is initiated; this complication is manifested in a considerably longer operating time than that required to produce couplers and/or splitters between standard fibres, and in a considerably lower efficiency of the process. These couplers are therefore expensive.
The Applicant has tackled the problem of producing couplers between a polarization maintaining fibre and a standard fibre with a simpler and relatively inexpensive technology. The Applicant has observed, however, that a fusion process of the standard type does not permit coupling between a polarization maintaining fibre and a standard fibre.
U.S. Pat. No. 5,293,440 describes a coupler for polarization maintaining fibres produced by the fusion technology, including a rigid substrate and two optical fibres placed adjacent to each other and joined together in their portions in which the coupling takes place.
Each optical fibre is fixed separately by an adhesive to the rigid substrate at each of its ends. The coupling portion of the two fibres can be formed by fusion or any other method of joining the two fibres together. The optical fibres of the coupler can be polarization maintaining fibres, single-mode optical fibres, multimode optical fibres. or their combinations. In one embodiment in this patent, a coupler called a hybrid coupler is described, in which coupling takes place between a polarization maintaining fibre and a single-mode optical fibre.
The Applicant has observed that, according to the aforesaid patent, even in a fusion coupler between a polarization maintaining fibre and a standard fibre, one of the axes of birefringence of the polarization maintaining fibre has to be aligned with the geometrical axis of the coupler, defined as the axis passing through the centres of the two fibres.
The Applicant has tackled the problem of the difference between the propagation constants of the two optical fibres fused together, where one is a polarization maintaining fibre and one is a standard fibre, which varies according to the value of the angle between the geometrical ads of the coupler and one of the two axes of birefringence of the polarization maintaining fibre.
The Applicant has found a method which can be used to produce an optical. coupler/splitter between a polarization maintaining fibre and a standard fibre by the fused fibre technology, used in the case of xe2x80x9cstandardxe2x80x9d fibres, using a process which does not require the alignment of one of the axes of birefringence of the polarization maintaining fibre with the geometrical axis of the coupler, but which still makes it possible to establish an optimal condition of optical coupling between the fused fibres.
In particular, the Applicant has discovered that, by tapering the xe2x80x9cstandardxe2x80x9d fibre before fusion, by suitably selecting the extent of the pre-tapering and then tapering both fibres together during the stage of fusion at a predetermined fusion temperature, it is possible to ensure that the relative cross sections in the coupling area are such that the coupling condition is achieved and that the extinction ratio is greater than 20 dB at the output of the polarization maintaining fibre.
The method according to the present invention makes it possible to dispense with the procedure of rotational alignment of the axes of birefringence of the polarization maintaining fibre with the geometrical axis of the coupler; it is therefore a particularly convenient method in terns of simplicity, efficiency and performance obtainable from the completed device.
In a first aspect, the present invention relates to a method for producing a fused coupler, comprising a polarization maintaining fibre and a standard fibre, comprising the following stages:
reducing the cross section of the standard fibre in such a way as to obtain a propagation constant for the standard fibre which differs from a propagation constant of the polarization maintaining fibre by less than a predetermined quantity corresponding to a condition permitting coupling between the fibres;
bringing the two fibres into contact with each other;
tapering the two fibres together and fusing a portion of each fibre, in a region where the two fibres are in contact with each other, at a fusion temperature which is higher than a predetermined temperature, where this predetermined temperature is such as to provide an extinction ratio of at least 20 dB at the output of the polarization maintaining fibre In case of a linear polarization at the input of the polarization maintaining fibre.
Preferably, the stage of reducing the cross section of the standard fibre comprises the stage of reducing the diameter of the said fibre.
In particular, the stage of reducing the standard fibre comprises the stage of tapering the said fibre by elongation.
Alternatively, the stage of reducing the standard fibre comprises the stage of reducing the diameter of the cladding while keeping that of the core unchanged.
Preferably, the said predetermined temperature is 1610xc2x0 C.
Preferably, the said fusion temperature is lower than 1650xc2x0 C.
In particular, the said propagation constant for the standard fibre differs from the propagation constant of the polarization maintaining fibre by less than a predetermined quantity corresponding to a coupling ratio of 1%.
In a further aspect, the present invention relates to a fused optical coupler comprising a polarization maintaining fibre and a standard fibre, wherein the cross section of the said standard fibre is smaller than the cross section of the polarization maintaining fibre in the area of fusion of the coupler in such a way as to obtain a propagation constant for the standard fibre which differs from a propagation constant of the polarization maintaining fibre by less than a predetermined quantity corresponding to a condition permitting coupling between the fibres, and the internal forces in the coupling area are sufficiently low to provide an extinction ratio of more than 20 dB at the output of the polarization maintaining fibre in case of a linear polarization at the input of the polarization maintaining fibre.